Synoptic and Local Influences on a Summertime, Long-lived, Mixed-phase Cloud Event over Summit, Greenland

Mallory Row

12 July 2016, 2:00 PM

Long-lived, Arctic mixed-phase clouds play a crucial role in modulating the surface energy balance over the Greenland Ice Sheet (GIS). However, little is known about the mechanisms that owe to their persistence due to infrequent observations. A theoretical framework by Morrison et al. 2012 hypothesizes that several local processes, such as cloud-top longwave radiative cooling, surface fluxes, and turbulent vertical motions, work together to drive a mixed-phase cloud system into a quasi-steady, persistent state. However, perturbations to this state by the synoptic environment can disrupt the balance between these local processes and lead to cloud dissipation. This research expands upon this framework with the hypothesis that local-scale processes can drive a cloud’s persistent state, while the synoptic-scale processes influence the thermodynamic structure of the lower troposphere, ultimately impacting the processes on the local scale.

A steady, single-layer, low-level, mixed-phase cloud was observed from 20-24 July 2012 at the Integrated Characterization of Energy, Clouds, Atmospheric State and Precipitation at Summit (ICECAPS) cloud-atmosphere observatory at Summit, Greenland. The Advanced Research Weather Research and Forecasting (WRF-ARW) model with polar modifications (PWRF) with Global Forecast System (GFS) final reanalysis data as initial and boundary conditions is used here in a series of controlled experiments to examine this study’s hypothesis on this mixed-phase cloud event. First, the role of the synoptic-scale processes is examined by fixing the boundary conditions to isolate the influence of the large-scale flow. Second, the role of the local processes is examined by modifying the cloud radiative forcings. It is found that synoptic-scale moisture advection provided the necessary moisture for the cloud to be maintained by local-scale processes, particularly cloud-top longwave radiative cooling and its induced feedbacks. Additionally, the cloud dissipation coincides with an air mass change as a result of synoptic-scale advection bringing colder and drier air over Summit.